The Denon and the Marantz both use the same video chipset UniPhier, (as does the Panasonic BD30) so I would expect their video performance to be similar, unless there was something wrong with yours. (BTW, Costco has the BD30 for $359.) They also both have the same realta post-processor; so it doesn't surprise me that they are basically the same player, probably even made in the same facility, just branded differently.

The Sony S500 uses an older chipset, Sigma SMP8634, which was also used in the (now discontinued) Panasonic BD10.